Place dried porcini mushrooms and 250 (1 cup) boiling water in a heatproof bowl. Cover with plastic wrap and soak for 20 minutes or until soft. Strain through a fine sieve into a bowl, reserving porcini liquid. Rinse porcini mushrooms under cold running water to remove any grit, then finely chop and transfer to a large bowl.

Meanwhile, heat 1 1/2 tbs oil in a large frying pan over a high heat. Cook half the field and Swiss brown mushrooms, tossing occasionally, for 6 minutes or until golden, then add to porcini. Repeat with remaining 1 1/2 tbs oil and mushrooms. Add prosciutto, truffle paste and parsley, seasono with salt and pepper and stir to combine.

To make white sauce, combine milk, cream, nutmeg, bay leaf and 125ml (1/2 cup) reserved porcini liquid in a saucepan, then bring lamost to the boil over medium heat. Remove from heat. In a second saucepan, melt 100g butter over medium heat, then stir in flour and cook, stirring, for 2 minutes or until mixture is grainy.

Remove pan from heat, then gradually whisk in milk mixture until smooth and combined. Return pan to medium heat and cook, stirring, for a further 5 minutes or until thickened. Discard bay leaf, then season. Makes about 1L.

Stir half the white sauce into the mushroom mixture, then spread half the remaining white sauce over the base of the prepared dish. Cover with a layer of lasagne sheets, cutting to fit if necessary. Spoon over half the mushroom mixture, then scatter with one-quarter of the Parmesan (and some truffle salt if using that option).

Add another layer of lasagne sheets, the remaining mushroom mixture and one-quarter of the Parmesan (and some truffle salt if using that option).

Top with a layer of lasagne sheets, the remaining white sauce and one-quarter of the Parmesan. cut remaining 20g butter into small cubes and scatter over the top.

Bake lasagne for 1 hour or until top is golden (cover dish with foil if over-browning). Cool for 10 minutes before slicing. Divide lasagne among plates, scatter over remaining Parmesan, drizzle with extra oil and season with pepper to serve.